An optical fiber is conventionally constituted of an optical core, which transmits an optical signal, and of an optical cladding, which confines the optical signal within the optical core. To that end the refractive index of the core, nc, is greater than the one of the cladding, ng. An optical fiber is generally characterized by a refractive index profile that associates the refractive index (n) with the radius (r) of the optical fiber: the distance r with respect to the center of the optical fiber is shown on x-axis and the difference between the refractive index at radius r and the refractive index of the optical cladding is shown on y-axis.
Nowadays, two main categories of optical fibers exist: multimode fibers and single-mode fibers. In a multimode fiber, for a given wavelength, several optical modes are propagated simultaneously along the optical fiber, whereas in a single-mode fiber, the higher order modes (hereafter called HOMs) are cut-off or highly attenuated.
Single-mode fibers are commonly used for long-distance applications, such as access networks. To obtain an optical fiber capable to transmit a single-mode optical signal, a core with a relatively small diameter is required (typically between 5 μm and 11 μm). To meet requirements of high speed or bit-rate applications (for example 10 Gbps), standard single-mode fibers require use of a modulated single-mode laser emitter tuned to work typically at a wavelength of 1550 nm.
Multimode fibers are commonly used for short-distance applications requiring a high bandwidth, such as local area networks (LANs) and multi-dwelling units (MDUs), more generally known as in-building networks. The core of a multimode fiber typically has a diameter of 50 μm, or 62.5 μm. The most prevalent multimode fibers in telecommunications are the refractive graded-index profile optical fibers. Such a refractive index profile guaranties, by minimizing the intermodal dispersion (i.e. the difference between the propagation delay times or group velocity of the optical modes along the optical fiber), a high modal bandwidth for a given wavelength.
For the development of an optical home network, the choice of the category of optical fiber category is critical. Multimode fiber is a cost effective solution for optical data networks. Thanks to their wider numerical aperture and core diameter, and their low modal dispersion provided by their graded-index core profile, multimode fibers can support efficiently 10 Gbps optical signals emitted by cost effective light sources based solutions (such as Vertical Cavity Surface Emitting Laser or VCSEL), whereas single-mode fibers require expensive and tolerant single-mode transceivers. In particular, the connection of the light source with the single-mode fiber (launching conditions) requires tighter alignment tolerances than with the multimode fiber.
However, since the optical home network is expected to be connected to outside access networks, which mainly use single-mode technology because of longer reach requirements, the problem of interoperability with single-mode fibers needs further considerations.
In practice, multimode fibers are not designed to be interconnected with single-mode transmission systems. A home network can be seen as a network of optical fibers that enables the users to connect devices at both ends of the network. Today, the devices are likely to implement multimode optical transmission based technologies that require multimode fibers, whilst tomorrow they could be designed to operate also with a single-mode based technology.
It is therefore desirable to provide a hybrid optical fiber for a home network that can transmit both multimode optical signals at an operating wavelength of the home network, for example 850 nm, and single-mode optical signals at an operating wavelength of an access network, for example 1550 nm, with an adequate trade-off of optical properties.
A known solution would consist in using a standard multimode fiber that has a refractive graded-index profile optimized for providing error-free transmission with a broad bandwidth at a wavelength of 850 nm. Nevertheless, when a single-mode source operating at a wavelength of 1550 nm is coupled to the standard multimode fiber, the optical signal injected in the fiber stimulates, mainly but unfortunately not only the fundamental optical mode, but also the HOMs within the optical fiber. These HOMs induce modal noises that degrade the quality of optical transmission. There are actually two main categories of modal noises: incoherent and coherent noises.
Incoherent noise is based on the fact that, on the emitter side, the optical signal coupled into the HOMs of the fiber may suffer from modal dispersion, and so since the different modes have different propagation delay times and propagation constants, these HOMs may degrade the quality of optical transmission by overlapping delayed copies of the main optical signal on the receiver side. In practice, to perform well in a high-bandwidth application, an optical fiber should have the highest quality of optical transmission, which can be measured by means of signal-to-noise ratio. For the incoherent contribution, the signal-to-noise ratio, hereafter called “signal to incoherent noise ratio”, can be defined by the following equation:
                              SNR          incoherent                =                                                          γ                                      4                                              ∑              i                                                                    ⁢                                                                            β                  i                                                            4                                                          (        I        )            wherein:|γ|2 is the optical power coupled into the fundamental mode;|βi|2 is the optical power coupled into the higher order modes (HOMs), with i≧1.
Coherent noise is based on the fact that, the optical signal coupled into the HOMs of the fiber on the emitter side may generate phase mismatch with the optical signal coupled into the fundamental mode, leading to uncontrolled interferometric recombinations into the fundamental mode on the receiver side. These interferences induce optical power fluctuation that also degrades the quality of optical transmission. For the coherent contribution, the signal-to-noise ratio, hereafter called “signal to coherent noise ratio”, can be defined by the following equation:
                              SNR          coherent                =                              (                                                                              γ                                                  4                            +                                                Σ                  i                                ⁢                                                                                                β                      i                                                                            4                                                      )                                σ            coherent                                              (        II        )            wherein:|γ|2 is the optical power coupled into the fundamental mode;|βi|2 is the optical power coupled into the higher order modes (HOMs), with i≧1;σcoherent is a standard deviation coefficient of a Gaussian noise.
As a result, when less the optical power is coupled into the HOMs, the optical transmission quality of the optical fiber is improved.
Due to the presence of these modal noises, such a standard multimode fiber is therefore not adapted to an interconnection with a single-mode optical transmission system.
It would be therefore efficient to provide an optical fiber having a broad modal bandwidth at a wavelength of 850 nm and a significant reduced level of modal noises at a wavelength of 1310 nm or 1550 nm.
The Australian patent document AU 2002/100296 discloses an optical fiber comprising a single-mode core portion, which has a first refractive index, surrounded by a multimode core portion, which has a second refractive index, finally surrounded by a cladding which has a third refractive index. The multi-portion index profile is arranged so that the fundamental mode is substantially matched to those of a single mode fiber. However, this document does not provide any solution for minimizing modal noises caused by the HOMs of the optical fiber. The disclosed optical fiber further presents a relatively low modal bandwidth at 850 nm and requires a complex index profile design.
The French patent document FR 2 441 858 discloses an optical fiber with a central single-mode core and a multimode sheath for data transmission. In particular, the disclosed optical fiber does not exhibit a graded-index profile (the multimode fiber portion has a step-index profile), which does not allow meeting the requirements in terms of high modal bandwidth at 850 nm. Nor does not address the problem of reduction of modal noises at a wavelength of 1310 nm or 1550 nm.
A solution to the problem of modal noises would be to reduce the core diameter of the multimode fiber. However reducing the optical core diameter leads to degrade the quality of multimode optical transmissions. Indeed, when a connection is carried out with a standard optical fiber (i.e. a fiber having a diameter of 50 μm or 62.5 μm), optical transmission losses are even more important where the core diameter is low, thereby significantly limiting the modal bandwidth of the optical fiber for multimode optical transmissions. Therefore such a solution is not optimal.